/*
 * Copyright (C) 2009 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.base.common.base;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;

import com.base.common.base.Service.State;

/**
 * An object with an operational state, plus asynchronous {@link #start()} and
 * {@link #stop()} lifecycle methods to transfer into and out of this state.
 * Example services include webservers, RPC servers and timers. The normal
 * lifecycle of a service is:
 * <ul>
 * <li>{@link State#NEW} -&gt;</li>
 * <li>{@link State#STARTING} -&gt;</li>
 * <li>{@link State#RUNNING} -&gt;</li>
 * <li>{@link State#STOPPING} -&gt;</li>
 * <li>{@link State#TERMINATED}</li>
 * </ul>
 * 
 * If the service fails while starting, running or stopping, its state will be
 * {@link State#FAILED}, and its behavior is undefined. Such a service cannot be
 * started nor stopped.
 * 
 * <p>
 * Implementors of this interface are strongly encouraged to extend
 * {@link com.google.common.util.concurrent.AbstractService} or
 * {@link com.google.common.util.concurrent.AbstractExecutionThreadService},
 * which make the threading and state management easier.
 * 
 * @author Jesse Wilson
 * @since 1
 */

public interface Service {
	/**
	 * If the service state is {@link State#NEW}, this initiates service startup
	 * and returns immediately. If the service has already been started, this
	 * method returns immediately without taking action. A stopped service may
	 * not be restarted.
	 * 
	 * @return a future for the startup result, regardless of whether this call
	 *         initiated startup. Calling {@link Future#get} will block until
	 *         the service has finished starting, and returns one of
	 *         {@link State#RUNNING}, {@link State#STOPPING} or
	 *         {@link State#TERMINATED}. If the service fails to start,
	 *         {@link Future#get} will throw an {@link ExecutionException}, and
	 *         the service's state will be {@link State#FAILED}. If it has
	 *         already finished starting, {@link Future#get} returns
	 *         immediately. Cancelling the returned future is unsupported and
	 *         always returns {@code false}.
	 */
	Future<State> start();

	/**
	 * Initiates service startup (if necessary), returning once the service has
	 * finished starting. Unlike calling {@code start().get()}, this method
	 * throws no checked exceptions.
	 * 
	 * @throws InterruptedRuntimeException
	 *             if the thread was interrupted while waiting for the service
	 *             to finish starting up.
	 * @throws RuntimeException
	 *             if startup failed
	 * @return the state of the service when startup finished.
	 */
	State startAndWait();

	/**
	 * Returns {@code true} if this service is {@link State#RUNNING running}.
	 */
	boolean isRunning();

	/**
	 * Returns the lifecycle state of the service.
	 */
	State state();

	/**
	 * If the service is {@link State#STARTING} or {@link State#RUNNING}, this
	 * initiates service shutdown and returns immediately. If this is
	 * {@link State#NEW}, it is {@link State#TERMINATED terminated} without
	 * having been started nor stopped. If the service has already been stopped,
	 * this method returns immediately without taking action.
	 * 
	 * @return a future for the shutdown result, regardless of whether this call
	 *         initiated shutdown. Calling {@link Future#get} will block until
	 *         the service has finished shutting down, and either returns
	 *         {@link State#TERMINATED} or throws an {@link ExecutionException}.
	 *         If it has already finished stopping, {@link Future#get} returns
	 *         immediately. Cancelling this future is unsupported and always
	 *         returns {@code false}.
	 */
	Future<State> stop();

	/**
	 * Initiates service shutdown (if necessary), returning once the service has
	 * finished stopping. If this is {@link State#STARTING}, startup will be
	 * cancelled. If this is {@link State#NEW}, it is {@link State#TERMINATED
	 * terminated} without having been started nor stopped. Unlike calling
	 * {@code stop().get()}, this method throws no checked exceptions.
	 * 
	 * @throws InterruptedRuntimeException
	 *             if the thread was interrupted while waiting for the service
	 *             to finish shutting down.
	 * @throws RuntimeException
	 *             if shutdown failed
	 * @return the state of the service when shutdown finished.
	 */
	State stopAndWait();

	/**
	 * The lifecycle states of a service.
	 * 
	 * @since 1
	 */

	enum State {
		/**
		 * A service in this state is inactive. It does minimal work and
		 * consumes minimal resources.
		 */
		NEW,

		/**
		 * A service in this state is transitioning to {@link #RUNNING}.
		 */
		STARTING,

		/**
		 * A service in this state is operational.
		 */
		RUNNING,

		/**
		 * A service in this state is transitioning to {@link #TERMINATED}.
		 */
		STOPPING,

		/**
		 * A service in this state has completed execution normally. It does
		 * minimal work and consumes minimal resources.
		 */
		TERMINATED,

		/**
		 * A service in this state has encountered a problem and may not be
		 * operational. It cannot be started nor stopped.
		 */
		FAILED
	}
}
